The present invention relates to pressure sensors and, more particularly, to pressure sensors having span compensation customized for a specific operating temperature range where the customization can be changed for a different specific operating temperature range.
Piezo-resistive pressure sensors are used in a wide variety of applications including automotive, industrial, medical, and environmental applications. Such sensors typically include a silicon diaphragm incorporating an ion implanted piezo-resistive Wheatstone bridge. An applied pressure bends the diaphragm and imbalances the bridge, producing a differential, ratiometric output signal that is proportional to the product of the change in resistance (xcex94R/R) caused by the pressure and the bridge excitation voltage.
Piezo-resistive pressure sensors formed in silicon are frequently fabricated using either bulk silicon or silicon-on-insulator (SOI) wafers. In the case of bulk silicon, the piezo-resistive pressure sensors are fabricated directly in the bulk silicon. In the case of SOI, the piezo-resistive pressure sensors are fabricated in the top silicon layer that is over the buried oxide layer of the SOI structure.
In the bridge configuration of piezo-resistive pressure sensors, the resistance of diagonally opposed legs varies equally and in the same direction as a function of the mechanical deformation caused by pressure. As the resistance of one set of diagonally opposed legs increases under pressure, the resistance of the other set decreases, and vice versa.
Bridge excitation in the form of a voltage or current is applied across two opposite nodes of the bridge. These nodes are usually referred to as excitation inputs or bridge drive inputs. Any change in resistance (such as due to a pressure input) is detected as a voltage difference across the other two nodes of the bridge, which are typically referred to as the bridge output.
For silicon based piezo-resistive pressure sensors, the voltage difference across the bridge output is quite small. Therefore, the bridge output is processed through a pressure channel that typically includes an amplifier to amplify the voltage difference across the bridge output. A feedback resistor around the amplifier is used to control the gain of the pressure channel that includes the amplifier. This feedback resistor is usually a light implant resistor compared to the heavy implant piezo-resistors that form the sensor bridge.
The full span output is defined as the difference in sensor outputs corresponding to the maximum and minimum applied pressures. The full span output (FSO) of an uncompensated piezo-resistive sensor bridge can exhibit a strong nonlinear dependence on temperature caused be the intrinsic nonlinear dependence of the piezo-resistor gauge factor (xcex94R/R) on temperature, whereas the zero pressure (null) offset and null offset dependence on temperature are maintained small in comparison. Thus, a sensor is typically compensated so that it can be used in practice.
Span shift with temperature is defined as the span as a function of temperature divided by the span at 25xc2x0 C. Accordingly, span shift(T) in percent is equal to 100xc2x7[Span(Txc2x0 C.)/Span(25xc2x0 C.)]. The span shift curve is nonlinear with a negative slope with temperature as illustrated in FIG. 4 and is identified as K3. The span shift K3 is defined as the ratio of the pressure sensitivity (xcex94R/R)) of the heavy implant piezo-resistive bridge as a function of temperature normalized to the value at 25xc2x0 C. In equation form K3(T) is equal to [xcex94R/R(T)]/[xcex94R/R(25xc2x0 C.)] and may be expressed by the following 5th order polynomial:
K3(T)=xe2x88x92(6.265753Exe2x88x9214)xc2x7T{circumflex over ( )}5+(5.393845Exe2x88x9211)xc2x7T{circumflex over ( )}4xe2x88x92(2.440481Exe2x88x9208)xc2x7T{circumflex over ( )}3+(8.022881Exe2x88x9206)xc2x7T{circumflex over ( )}2xe2x88x92(2.585262Exe2x88x9203)xc2x7T+(1.058300)
The magnitude of the slope decreases with increasing temperature. A typical span shift value at 25xc2x0 C. is xe2x88x920.25%xc2x0 C. Thus, in most applications, the sensor bridge output must be compensated, for the span shift(T) in particular, before it can be used in practice.
FIG. 4 also illustrates the temperature characteristics of the heavy implant resistors (K2) and the light implant resistors (K1). The temperature characteristics of the heavy implant resistors (K2) is defined as follows: K2=Ratio of the resistance of the heavy implant resistor as a function of temperature normalized to the value at 25xc2x0 C. In equation form, K2=[Rheavy(T)]/[Rheavy(25xc2x0 C.)] and may be expressed by the following 5th order polynomial:
K2(T)=xe2x88x92(3.018497Exe2x88x9214)xc2x7T{circumflex over ( )}5+(4.603604Exe2x88x9211)xc2x7T{circumflex over ( )}4xe2x88x92(2.282857Exe2x88x9208)xc2x7T{circumflex over ( )}3+(7.538750Exe2x88x9206)xc2x7T{circumflex over ( )}2xe2x88x92(2.252834Exe2x88x9205)xc2x7T+(0.9963789)
The temperature characteristics of the light implant resistors (K1) is defined as follows: K1=Ratio of the resistance of the light implant resistor as a function of temperature normalized to the value at 25xc2x0 C. In equation form, K1(T)=[Rlight(T)]/{Rlight(25xc2x0 C.)] and may be expressed by the following 5th order polynomial:
K1(T)=xe2x88x92(8.171496Exe2x88x9214)xc2x7T{circumflex over ( )}5+(9.930398Exe2x88x9211)xc2x7T{circumflex over ( )}4xe2x88x92(3.557091Exe2x88x9208)xc2x7T{circumflex over ( )}3+(9.691127Exe2x88x9206)xc2x7T{circumflex over ( )}2+(2.958093Exe2x88x9203)xc2x7T+0.923953
It is noted that the change in resistance of light implant resistors as a function of temperature K1(T) is much greater than that of the heavy implant resistors (K2(T).
SOI piezo-resistive pressure sensors that use the same design layout and the same ion implant concentrations as that used to fabricate bulk silicon piezo-resistive pressure sensors experience a positive span shift in the amplified pressure channel output as a function of temperature, whereas the span shift of the SOI piezo-resistive bridge-only has a negative slope with temperature, which is similar to that of bulk piezo-resistive sensors. This comparison indicates that the bridge span shift has been over-compensated by the positive temperature dependant gain of the amplifier. This over-compensation of span shift is primarily due to the higher positive TCR (temperature coefficient of resistance) of the light implant (high TCR) resistor elements in the SOI construction as used in the gain and feedback resistor networks which in turn causes an increase in the temperature dependent gain of the amplifier. As a result, the amplified pressure sensor output span is changed from being under-compensated to being over-compensated as shown in FIG. 5.
For example, in bulk silicon devices, the positive TCR of the light implant feedback (gain) resistor in the pressure channel compensates approximately 85% of the negative temperature coefficient (TC) of the heavy implant pressure bridge""s span shift, as shown by the curve with the delta points, which results in the amplified output to be under-compensated by approximately 15% as shown by the curve with the square points. However, for SOI devices, the increase in the TCR of the light implant gain resistor causes an over-compensation of span shift with temperature as shown by the curve with the circle points. As shown in FIG. 5, the approximate +23% span shift of the bridge output (the curve with the delta points) at xe2x88x9255xc2x0 C. is over-compensated to approximately xe2x88x923% at the amplified output. Similarly, the approximate xe2x88x9228% span shift of the bridge output at +225xc2x0 C. is over-compensated to approximately +7% at the amplified output.
Known techniques have been implemented to reduce the effect of span shift over-compensation. For example, in one technique, the dopant concentration of the SOI light implant process can be reduced to provide a lower TCR and a lower sheet resistivity. This process requires extensive re-layout of all of the light implant elements, processing of product for verification, and the need to have a third implant variable in the production wafer fabrication.
In another technique, the effective TCR of all of the light implant feedback and bias resistors in the pressure channel is reduced by making a certain percentage of these elements heavy implant elements. This process requires extensive re-design and re-layout of all the feedback and bias resistors, and could jeopardize the exact tracking of the Rfeedback/Rbias ratio required for high performance and long term stability.
Accordingly, these known techniques are complex and are not practical.
The present invention optimizes span compensation that may be customized for a specific operating temperature range and permits re-customization for a different operating temperature range. A benefit of the present invention is that the same ion implant concentrations that are common to both bulk silicon and SOI pressure sensors can be retained.
According to one aspect of the present invention, a sensor chip has first, second, third, fourth, fifth, and sixth sensor nodes and comprises a piezo-resistive bridge and first, second, third, fourth, fifth, and sixth conductive paths. The piezo-resistive bridge has first and second input nodes and first and second output nodes. The first conductive path connects the first input node to the first sensor node. The second conductive path connects the second input node to the second sensor node. The third conductive path connects the first output node to the third sensor node. The fourth conductive path connects the second output node to the fourth sensor node, the fourth conductive path includes a leadout resistance, a portion of the leadout resistance is formed as a light implant resistance, and the remainder of the leadout resistance is formed as a heavy implant resistance. The fifth conductive path connects the fourth sensor node to the fifth sensor node, and the fifth conductive path includes a bias resistance. The sixth conductive path connects the fourth sensor node to the sixth sensor node, and the sixth conductive path includes a feedback resistance.
According to another aspect of the present invention, a sensing system comprises a resistive sensor bridge, an amplifier, and first, second, third, fourth, fifth, and sixth conductive paths. The resistive sensor bridge has first and second input nodes and first and second output nodes. The amplifier has negative and positive inputs and an output. The first conductive path connects the first input node to a first source. The second conductive path connects the second input node to a reference potential. The third conductive path connects the first output node to the positive input of the amplifier. The fourth conductive path connects the second output node to the negative input of the amplifier, the fourth conductive path includes a leadout resistance, a portion of the leadout resistance is formed as a light implant resistance, and the remainder of the leadout resistance is formed as a heavy implant resistance. The fifth conductive path connects a second source to the negative input of the amplifier, and the fifth conductive path includes a bias resistance. The sixth conductive path connects the output of the amplifier to the negative input of the amplifier, and the sixth conductive path includes a feedback resistance.
According to still another aspect of the present invention, a method of fabricating a sensor chip comprises the following: forming a resistive sensor having at least a first resistance and a second resistance, wherein the resistive bridge has first and second input nodes and at least one output node; forming a first conductive path connected to the first input node; forming a second conductive path connected to the second input node; and, forming a third conductive path connected to the output node, wherein the third conductive path comprises a third and fourth resistances, wherein each of the first, second, and third resistances is a heavy implant resistance, and wherein the fourth resistance is a light implant resistance.